U.S. Defense Secretary Ash Carter on Wednesday spelled out the U.S. strategy for retaking the Iraqi city of Mosul and Raqqa in Syria — cities he called Islamic State's two power centers.

Carter spoke to troops from the 101st Airborne Division in Fort Campbell, Kentucky, who will soon be deployed to Iraq to help train Iraqi and Kurdish forces in defeating the militants.

"ISIL [Islamic State] is a cancer that's threatening to spread," Carter told them. "And like all cancers, you can't cure the disease just by cutting out the tumor. You have to eliminate it wherever it has spread and stop it from coming back."

Carter said he envisioned sending Iraqi forces from the south and Kurdish Peshmerga fighters from the north to surround Mosul and cut it off.

U.S. troops will also help train Syrian rebel forces in ousting Islamic State from Raqqa in Syria, a city the militants call their capital.

Carter said the U.S. special operations forces deployed to Iraq in December were ready to go to work assisting the Iraqis.

The expeditionary force "is now in place and is preparing to work with the Iraqis to begin going after ISIL's fighters and commanders, killing or capturing them wherever we find them, along with other key targets," he said.

A U.S. airstrike earlier this week hit a building in Mosul that was storing Islamic State cash, destroying millions of dollars in illicit funds.

Carter also said Wednesday that he would go to Paris next week to meet with the French, Australian, British, Dutch, German and Italian defense ministers.

He said that each one of these nations has a "significant stake" in destroying Islamic State and that the fight must include all the capabilities they can bring.